The present invention relates to a method of changing image magnification, and more particularly to a method of enlarging or reducing image information produced from a light sensor after it has read an original image as modified in size by optical means, at a desired magnification ratio with electric means, so that an image which is magnified at a desired ratio can be reproduced highly accurately and inexpensively.
In the field of printing plate-making, for example, there has been employed an image scanning recording and reproducing system for electrically processing image information on an original to produce an original film plate in order to carry out the plate-making process efficiently and improving the image quality.
The image scanning recording and reproducing system is basically composed of an image reading device and an image reproducing device. In the image reading device, image information on an original which is fed in an auxiliary scanning direction is scanned in a main scanning direction by a light sensor, which converts the image information to an electric signal. Then, the photoelectrically converted image information is subjected to gradation correction, profile emphasis, and other image processing in the image reproducing device according to given plate-making conditions. Thereafter, the image information is converted to a light signal such as a laser beam signal which is applied to a recording medium comprising a photosensitive material such as a photographic film to record the image information thereon. The image on the recording medium is developed by an image developing device, and the recording medium will be used as a film plate in a printing process.
It is preferable that the image information carried on the original be reproduced at an enlarged or reduced size by selecting a desired magnification ratio. One method of changing the magnification ratio of image information is to use a zoom lens in the optical system for focusing the image information on the light sensor and vary the magnification ratio by operating the zoom lens.
A magnification ratio for image information can be changed as desired by the zoom lens. However, where image information is greatly enlarged by the zoom lens, the image information focused on the light sensor suffers from various optical defects such as spherical aberration, distortion, and the like, resulting in a reduction image accuracy. It would be technically difficult and highly expensive to produce a zoom lens having minimum aberrations and capable of magnifying image information in a wide magnification range.
It is also possible to employ a fixed-focus lens, instead of a zoom lens, and to displace the fixed-focus lens and the light sensor with respect to an original for thereby changing the magnification ratio.
With this method, however, the lens and the light sensor must be displaced greatly in order to greatly enlarge or reduce the original image, and a wide space is necessary to accommodate such a large displacement of the lens and the light sensor. Where the magnification of the original image is increased, optical defects or aberrations are produced to lower the image accuracy, as with the zoom lens.
An electric process may be employed for changing an image magnification ratio. For example, image information photoelectrically converted by a light sensor and stored in a memory may be reproduced in a 1/2 size by reading the stored information from every other address of the memory. Likewise, stored image information may be reproduced in a reduced size at a magnification ratio of 1/3, 1/4, . . . Conversely, an image may be reproduced at a magnification ratio which is a multiple of an integer by reading image information several times from each address of the memory.
It is however impossible to continuously change the magnification ratio to obtain magnification of x 1.3 or x 3.8, for example, since available magnification ratios are multiples of an integer. An enlarged reproduced image is not smooth and continuous because it is reproduced by reading the stored information several times from each address of the memory.